Segmentation of the caudal hindbrain into rhombomeres (r) 4-7 is regulated by paralog group 1 (PG1) hox proteins, but it is unclear how PG1 Hox proteins activate their target genes and how these genes subsequently interact to produce distinct rhombomeres. Our preliminary results indicate that Hoxblb (a zebrafish PG1 Hox protein) forms repressive complexes with Pbx proteins and histone deacetylases (HDACs) and that Meis proteins are required to convert Pbx:Hoxb1b complexes into transcriptional activators. We have also demonstrated that Hoxblb regulates the expression of several genes in r4 (e.g. hoxbla) and r5/r6 (e.g. val, vhnfl), but it is unclear how these genes act and it is likely that additional hoxblb target genes exist. We hypothesize that Meis proteins displace HDACs from Pbx:Hoxb1b complexes to activate transcription of a few target genes that in turn activate a cascade of transcription factors required for formation of r4-r7. Our first aim is to delineate hoxblb-regulated pathways that control formation of r4-r6. In particular, we will establish in which order vhnfl, val and hoxbl a act and how their expression domains become refined. We will also explore the function of several novel genes we have identified in r5/r6. Our second aim is to determine the role of Meis and Pbx cofactors in modulation of Hoxblb function. We will explore the mechanism whereby Meis proteins displace HDACs from Pbx and we will test whether Meis acts in a similar manner to displace HDACs during activation of Hoxbl b target genes in vivo. Our experiments are important for several reasons. First, the hindbrain gives rise to several important structures - sensory ganglia and branchiomotor neurons, as well as bone, cartilage and muscle of the vertebrate head. The developing hindbrain is sensitive to disruptions by a variety of factors (e.g. environmental toxins, infectious agents and genetic conditions) that give rise to a range of birth defects - motor control problems such as ataxia, cognitive defects such as autism and craniofacial defects. In addition, Hox proteins and Hox cofactors regulate other aspects of neural development (e.g. dorsoventral patterning of the neural tube), and other aspects of embryogenesis (e.g. hematopoiesis). hox, meis and pbx genes are also protooncogenes involved in leukemia. A better understanding of Hox function will therefore be applicable to a broad set of biological processes and human disease conditions.